Renewable energy sources such as wind or solar energy for example have become viable alternatives in recent years, appropriate for addressing global warming and the depletion of fossil fuel resources. In addition, the development of transportation with low environmental impact, such as the electric tram, train, or car, relies on the development of suitable power electronics, and more specifically power switches.
In particular, improvements to semiconductor components in the form of integrated circuits, for example such as power transistors, concern the intrinsic properties of these components for increasing their operating voltage and/or their maximum switching frequency. They also aim to provide solutions for total integration enabling mass production, in order to reduce production costs.
At this time, only power components based on silicon (Si) fulfill such functions, for example such as MOS (“Metal Oxide Semiconductor”) or IGBT (“Insulated Gate Bipolar Transistor”) transistors. However, the physical properties inherent to the use of Si make technological advancements difficult in such components.
In recent years, numerous research projects have led to finding alternative solutions with the use of wide-bandgap materials, enabling the implementation of new power components such as high electron mobility transistors, also called HEMT transistors.
Silicon carbide (SiC) and gallium nitride (GaN) have emerged as two of the most promising materials due to their high critical electric field and a wide range of operating temperatures. Despite remarkable results, however, power components based on SiC have difficulties penetrating mass markets due to the limited size of SiC wafers (maximum diameter of 100 mm to 150 mm at present). In addition, problems remain for this type of component, concerning the management of defects as well as the reproducibility of the manufacturing methods.
GaN seems to be a very attractive alternative to SiC for the design of the power components. GaN is a more effective semiconductor material than Si or SiC in terms of compromise between on-resistance/breakdown voltage. This ratio, also known as figure-of-merit, characterizes the static performance of a power switch.
FIG. 1 shows an exemplary electronic heterojunction structure used in an HEMT transistor. This electronic heterojunction structure comprises several layers based on GaN, each having controlled intrinsic properties and stacked one above the other, with:
a substrate 1,
above which lies a first layer 3, called the buffer layer, composed of a material M1 characterized by its bandgap or gap Eg1; and,
a second layer 5, called the barrier layer, above the first layer 3 and composed of a material M2 characterized by its bandgap or gap Eg2, where Eg1 is less than Eg2.
Many research projects focus on the management and improvement of two-dimensional electron gas confinement at the AlGaN/GaN heterostructure, investigating various solutions. These solutions may involve deposition of the GaN layer so as to modify certain intrinsic properties of the heterostructure, thereby obtaining HEMT transistors with relatively high switching speeds and relatively low losses. They may also consist of creating new MOS-HEMT type structures.
Thus, for example, a MOS-HEMT transistor structure is proposed in the 2008 IEEE publication: “Enhanced device performance of AlGaN/GaN HEMTs using thermal oxidation of electron-beam deposited aluminum for gate oxide” by C. Hongwei et al. This publication shows the improvement in the performance of a conventional HEMT transistor structure which can be obtained by adding an oxidation layer at the gate electrode. The resulting MOS-HEMT structure has smaller leakage currents and a greater range of drain currents than a conventional HEMT structure, although a threshold voltage of less than zero volts is required to place the transistor in an off state.
Yet another area of very significant research currently concerns the resting state of this type of structure, meaning the state of the transistor when no voltage is applied to the gate electrode of the HEMT transistor. In fact, in many power applications, the transistor used as a switch must be in the open state by default (also called the “normally OFF” functionality). This state is essential for safety reasons and to save energy, for example in automotive or railway applications.
Several GaN-based structures have recently been proposed to offer the “normally OFF” functionality of an HEMT transistor. Work by the team of C. Hongwei et al has demonstrated the possibility of modifying the threshold voltage to obtain a “normally OFF” HEMT transistor by using treatment with fluorine ions, in a publication called “Self-aligned enhancement-mode AlGaN/GaN HEMTs Using 25 keV Fluorine Ion Implantation” published in 2010 in the IEEE journal. To do this, a fluorine-doped area is inserted into the barrier layer of the AlGaN/GaN structure of the HEMT transistor and this is placed below the gate electrode, the doses of fluorine ion being predefined so as to have a sufficient difference in the voltage Vgs of the transistor.
US patent application 2007/0278518 entitled “Enhancement Mode III-N Devices and Circuits” proposes another enhancement to the manufacturing method for an HEMT transistor structure. According to this enhancement, a method of treating the barrier layer of the heterostructure by a fluorine plasma is used. By a relatively simple process (using a fluorine plasma), this method modifies the intrinsic properties of the heterostructure in order to obtain a “normally OFF” transistor.
Advancements in design and manufacturing techniques such as those presented above allow obtaining “normally OFF” HEMT transistors which can only address certain energy conversion markets, due to the still relatively high leakage currents.
US patent application 2013/0256685 entitled “Compound semiconductor device and method for manufacturing the same” proposes an HEMT-based structure in which a two-dimensional electron gas is generated, and an electrode is formed on the HEMT-based structure. The structure further comprises a P-type semiconductor layer below a region where the two-dimensional electron gas is generated. To control the electron density of the two-dimensional gas, the P-type semiconductor layer includes a portion containing a larger amount of ionized acceptors than other portions of the P-type semiconductor layer.